1. Field of the Invention
Aspects of the disclosed apparatus and methods relate to providing a gradient strain to an extracellular matrix.
2. Description of the Related Art
Mechanical forces regulate the development and phenotype of a variety of tissues and cultured cells. Previous studies involving mechanotransduction use methods to apply mechanical strain to cells and tissues. These studies suggest that the cell's extracellular matrix (ECM) attachments are the sites at which forces are transmitted to cells. Hydrogels polymerized from natural, synthetic, or hybrid molecules are commonly used as ECM for the study of cell-ECM interactions as well as for medically implantable biomaterials and potential scaffolds for tissue regeneration. The design of a hydrogel that mimics the physiological microenvironment requires consideration of a multitude of factors including micromechanical properties, biocompatibility, ligand concentration, biotransport kinetics, and pore size. Complex interactions between these factors contribute to the transduction of cellular signals, which in turn determines cell survival, proliferation, and phenotype. Uncovering the exact role of stiffness in regulating cells in 3D has proven to be difficult because tuning stiffness in a physiologically relevant system is non-trivial. While the bulk mechanics of 3D matrices can be made effectively more stiff by increasing ECM protein concentration or altering the molecular weight of monomers, there is a resulting decrease in mesh pore size, and increase in cellular confinement, resistance to transport, and local concentration of ligand presented to cells cultured within. Protein-polymer hybrid systems such as PEG-fibrinogen or collagen-agarose allow one to tune stiffness independent from bulk ligand concentration. However, the mesh size of these systems is commonly much smaller than their naturally derived protein hydrogel counterparts, thus increasing both resistance to transport and cellular confinement as compared to naturally derived systems. While phenotypic changes have been demonstrated in such systems, their relevance is debatable in the context of understanding basic physiology.
Fibrin is a commonly used naturally occurring viscoelastic biopolymer. Fibrin is the polymerized form of the blood circulating protein fibrinogen, and is the predominant structural component of blood clots that form in response to injury. Fibrin hydrogels exhibit many interesting mechanical properties, including high extensibility and negative compressibility, all while maintaining permeability and bulk structural integrity under proteolytic degradation and cellular contraction, making it an ideal substrate for the wound healing process. The molecular basis for fibrin's remarkable physical behavior has been investigated at the scale of individual fibers, networks of fibers, and within macro-scale hydrogels. A more complete understanding of the role of fibrin's astounding mechanical properties in disease and thrombosis, as well as its function as a scaffold which drives tissue morphogenesis, will lead to better design strategies for tissue regeneration and engineering.